A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In dual stage lithography systems, which include two (dual stage) or more substrate tables, a substrate that is to be exposed with a pattern is analyzed at a measure side of the lithographic apparatus to obtain a substrate map. The substrate map relates to the surface profile of the substrate to be exposed in the lithographic apparatus. Such a substrate map is measured with a level sensor. Based on the substrate map, optimal leveling profiles are calculated, which are to be used during exposure to compensate for the surface profile of the substrate.
After measuring the substrate map, the substrate table is then moved from the measure side to an exposure side of the lithographic apparatus, to do the exposures. During the exposures the calculated leveling profiles are used to adjust the substrate table accordingly. The executed leveling profiles are adjusted to account for, e.g., the calibrated differences between the position sensors at the measure side and the expose side of the lithographic apparatus. For immersion systems, corrections are also made for the chuck deformation due to cooling of the wafer.
During exposure, the substrate table is subjected to relatively large acceleration forces in between the actual exposure scans, which are typically executed at constant velocity. Due to the large acceleration forces the position sensor associated with the substrate table may deform. Typically, this type of position sensor includes an interferometer equipped with a mirror block of one or more mirrors which may deform.
In the constant velocity exposure scans the acceleration forces are zero. However, acceleration forces may cause a small amount of slip between the substrate table and the mirror block. Therefore, the actual shape of the substrate table can be different for a substrate map scan in a first upward direction and a scan in a second downward direction opposite to the first direction (i.e., up-scan and down-scan). This can be regarded as a time-dependent hysteretic effect. A similar effect may occur for the substrate map measurements. As a result of the deformations, focus errors will be made during exposures.